Four Principles to Consider Before Advising Women on Screening Mammography

Abstract

This article reviews four important screening principles applicable to screening mammography in order to facilitate informed choice. The first principle is that screening may help, hurt, or have no effect. In order to reduce mortality and mastectomy rates, screening must reduce the rate of advanced disease, which likely has not happened. Through overdiagnosis, screening produces substantial harm by increasing both lumpectomy and mastectomy rates, which offsets the often-promised benefit of less invasive therapy. Next, all-cause mortality is the most reliable way to measure the efficacy of a screening intervention. Disease-specific mortality is biased due to difficulties in attribution of cause of death and to increased mortality due to overdiagnosis and the resulting overtreatment with radiotherapy and chemotherapy. To enhance participation, the benefit from screening is often presented in relative instead of absolute terms. Third, some screening statistics must be interpreted with caution. Increased survival time and the percentage of early-stage tumors at detection sound plausible, but are affected by lead-time and length biases. In addition, analyses that only include women who attend screening cannot reliably correct for selection bias. The final principle is that accounting for tumor biology is important for accurate estimates of lead time, and the potential benefit from screening. Since "early detection" is actually late in a tumor's lifetime, the time window when screen detection might extend a woman's life is narrow, as many tumors that can form metastases will already have done so. Instead of encouraging screening mammography, physicians should help women make an informed decision as with any medical intervention.

FIG. 1.

Incidence rate of breast cancer in the United States since screening mammography was introduced. Surveillance, epidemiology, and end results data for invasive breast cancers without metastases, with regional metastases, and those with distant metastases involving other organs at diagnosis, as well as ductal carcinoma in situ lesions (DCIS) are included, for women aged 40–84 years at diagnosis, apart from DCIS (50+ years). There was no decline in the occurrence of cancers with distant metastases over the observation period, a slight decline for cancers with regional metastases, and vast increases for both localized and in situ cancers.

FIG. 2.

Ten-year all-cause death risk for U.S. women by smoking status (age 40 and 50 years) compared with the breast cancer death risk and lives extended through screening. Starting at age 50, 7–13 women die from something else for every breast cancer death. The absolute risk reduction (ARR) or benefit from routine screening mammography is the assumed relative risk reduction (RRR) times the breast cancer death risk without screening. The ARR in absolute numbers is the same as lives extended through inviting 1000 women to routine screening.

FIG. 3.

Misleading statistics about screening benefits. The increased percentage of localized breast cancers (stages 0 and 1, <2 cm) after screen-detection does not prove screening is beneficial. This result is due to lead-time bias and length bias (overdiagnosis). The relevant statistic is the incidence rate of advanced cancers. Likewise, the increased percentage of smaller invasive tumors after screen-detection does not prove a morbidity or mortality benefit. Data is from the U.S. Breast Cancer Surveillance Consortium.

FIG. 4.

“Early” detection is late. On the left, each concentric circle depicts one volume doubling of an invasive breast tumor (time scale). The right graph shows the tumor size correlating to each volume doubling. Each shade codes for a specific step in tumor development important to screening mammography. The difference in median tumor size between screen-detected and clinically detected tumors (5–7 mm) is one to two of 32 tumor-doubling times. The tumor size difference is overestimated due to overdiagnosis.

FIG. 5.

Simplified screening mammography decision model with benefit and harms. The top branch, or no screening decision, shows that some early cancers do not progress to cause symptoms. With screening (bottom branch), many of these lesions are detected earlier, resulting in the harm of overdiagnosis. Although some otherwise lethal cases can be cured from earlier treatment, most cases that are not overdiagnosis remain curable or remain lethal despite screen-detection. Since mammography is imperfect, some cancers are not detected (false negatives), and some healthy women are recalled for additional testing and biopsy (false positives). Color images available online at www.liebertpub.com/jwh